Since the invention of relatively efficient fluorescent organic light-emitting diodes (OLEDs), a new generation of flat-panel displays has emerged with a potential for capturing a substantial market share of consumable electronics, such as television sets and computer monitors. While full-color OLED displays require the emission of blue, green and red light, white OLEDs are potentially useful as efficient and inexpensive solid-state lighting and as backlights for liquid crystal displays. Compared to molecular materials that can be vacuum-deposited into thin films, solution processible materials, such as π-conjugated polymers and monodisperse π-conjugated oligomers, offer cost advantage and ease of scale-up to large-area thin films.
Fluorescence or phosphorescence is responsible for light emission from organic luminophores. Electrophosphorescence is superior to electrofluorescence in terms of internal quantum yield, 100 versus 25%. Despite the intensive efforts worldwide over the past decade, device efficiency and lifetime have remained major challenges to both types of OLED. For the fabrication of an efficient phosphorescent OLED, a triplet emitter (a guest) is typically doped in a host material with a higher triplet energy, ET, to realize blue, green or red emission. To substantially improve device efficiency and lifetime, it is imperative that the electron and hole fluxes through fluorescent and phosphorescent OLED devices be balanced and to prevent the accumulation of charges and excitons at interfaces.
Normally, triplet host materials are capable of preferentially transporting holes or electrons. An electron- and/or a hole-transport layer is added to facilitate the injection and transport of deficient charge carriers into the emitter layer for efficient light emission. Nevertheless, with a unipolar host, charge recombination tends to occur close to the interface with the charge-transport layer for lack of bipolar-transport capability of emitter layer. Under the high current density associated with practical applications, confinement of excitons to the interfacial region could lead to fast triplet-triplet annihilation, resulting in efficiency roll-off. Furthermore, a narrow recombination zone is detrimental to operational stability because only a fraction of molecules contribute to charge transport, exciton formation, and light emission. Mixed hosts consisting of electron- and hole-transport molecules have been attempted to alleviate this problem. It has been demonstrated that mixed hosts can effectively decrease driving voltage while improving device efficiency sustainable at high current densities. A typical phosphorescent layer is comprised of a host mixed with a charge-transport component at 25 to 50 wt %, into which 1 to 10 wt % of triplet emitter is doped. The desired bipolar-transport capability requires a high concentration of the charge-transport additive, which may result in phase separation that can adversely affect long-term operational stability of OLEDs.
In general, the host materials should have a minimum ET level of 2.7, 2.5, and 2.0 eV to accommodate a blue-, green- and red-emitting guest, respectively. The HOMO/LUMO (highest occupied molecular orbital/lowest occupied molecular orbital) energy levels and charge-transport properties can be tuned by physically blending triplet hosts with appropriate materials at a risk of phase separation. Methods also exist for tuning ET by chemical modification assisted by computational chemistry. In principle, the host's ET level must be higher than the guest's to ensure host-to-guest energy transfer without the undesirable backward transfer, but close proximity of the two ET values is favorable to energy transfer because of the strong spectral overlap. The HOMO/LUMO levels can also be adjusted through chemical modification to facilitate the injection of holes and electrons.
A bipolar compound can be constructed by chemically bonding an electron- and a hole-transport moiety resulting in conjugated bipolar compounds. Conjugated bipolar compounds tend to be rigid and bulky, thus limiting solubility and the ability to form morphologically stable glassy films, which are essential to the fabrication of organic electronic devices for information display and solid-state lighting. Furthermore, as a result of the conjugation of the electron- and a hole-transport moieties, the ET of these moieties (triplet energy) are consistently diminished and LUMO/HOMO levels of these moieties are substantially modified in comparison to those of the two independent moieties, thus limiting the practical utility of conjugated hybrids.
Thus, based on the foregoing there exists an ongoing and unmet need for host materials with improved properties.